Gulf oil spill study sheds light on urban air pollution

When a team of researchers from the National Oceanic and Atmospheric Administration and the University of Colorado Boulder's Cooperative Institute for Research in Environmental Sciences raced to the scene of the BP Deepwater Horizon oil spill to assess the disaster's impact on air quality last year, they found more than they expected.

A significant fraction of the oil that rose to the water's surface had evaporated. Also, measurements taken onboard the NOAA WP-3D aircraft revealed that organic aerosols -- a form of air pollution -- formed from the oil vapors. Aside from the common culprits that create organic aerosols, the researchers discovered a new set of chemicals that contribute to diminished air quality -- chemicals that also exist in urban environments.

"It was very clear that the aerosols were formed from compounds not currently measured," said CIRES Fellow Joost de Gouw, lead author of a new paper published in the journal Science March 10. Discovering these previously unknown sources of aerosols could improve scientists' understanding of air pollution and how to regulate it in the future, said de Gouw. "This really shows that we need to start paying more attention to these compounds."

Aerosols are microscopic particles suspended in the air. In U.S. cities with air pollution about half of the pollution particles consist of organic material. Organic aerosols are linked to asthma, cardiovascular disease and even premature death. But scientists only know the origin of a small fraction of organic aerosols.

"The problem has been that we know there are more organic aerosols than we can account for," de Gouw said. "So there is a lot of discussion in the literature on where this organic material comes from."

The team's research on the air quality impacts of the oil spill shed new light on this mystery. In early June, a team of scientists from NOAA and CIRES arrived at the scene of the spill to assess how much of the oil was evaporating into the atmosphere, and whether the oil was a concern for air quality.

Using a Lockheed WP-3D Orion aircraft, the team flew for about 14 hours directly over and downwind of the oil spill. Instruments on board the aircraft measured many types of air pollution particles, including organic aerosols and the chemicals from which they are formed in the marine boundary layer -- the layer trapping most pollutants.

Based on the current understanding that the most volatile components of oil form air pollution particles, de Gouw and his colleagues knew where they expected to see the aerosols: exactly where they saw the most volatile components of the oil evaporate. "Our instrument showed a very narrow plume of oil compounds downwind from the spill site," said de Gouw -- so the scientists expected to see the organic aerosols in this same region.

But this is not what the scientists observed. "We detected particles being formed, but over a much wider area," said de Gouw. "So that was a big surprise."

The scientists realized that other compounds, aside from the highly volatile components of the oil, had to be contributing to the air pollution. Because they recorded organic aerosols over a broad area, they concluded the heavier, less volatile compounds that are slower to evaporate were also forming aerosols.

The lighter, highly volatile compounds evaporate quickly from a small area of the ocean and form air particles in that region only, de Gouw said. But heavier compounds have a chance to spread out before they evaporate, giving rise to the much wider band of organic aerosols that the team detected.

In 2007, other atmospheric scientists had proposed that heavier or "less volatile" components could in theory help to create organic aerosols. But it had proven to be nearly impossible to study the process in the real world, de Gouw said. "The problem is that volatile and less volatile species are emitted at the same time from the same combustion sources, so we could not study them separately in the atmosphere until Deepwater Horizon."

When de Gouw and his colleagues ran a series of models showing how spilled oil spread across the water, and how long it should take for various heavy, medium and light fractions to evaporate, the conclusion was clear: Heavier compounds from the oil that are slower to evaporate were the culprit.

The finding is not one that is specific to catastrophic oil spills, de Gouw said. The oil was not a thick sludge but more similar to the highly refined oil that is used in cars or factories, he said. That means the same heavier compounds that contributed to air pollution over the Gulf oil spill also contribute to air pollution in urban environments.

But these compounds are not measured in most air-quality monitoring programs designed to capture the conventional contributors to poor air quality. "This chemistry could be a very important source of aerosols in the urban United States and elsewhere," de Gouw said. "What we learned from this study will help us to improve air quality understanding and prediction."

Co-authors on the paper are from NOAA, CIRES, the University of Miami, the University of California, Irvine, the National Center for Atmospheric Research in Boulder and Carnegie Mellon University in Pittsburgh.